Proper cardiac function relies on the synchronized contractions of the heart at regular intervals. When normal cardiac rhythm is initiated at the sinoatrial node, the heart is said to be in sinus rhythm. However, when the heart experiences irregularities in its coordinated contraction, due to electrophysiologic disturbances caused by a disease process or from an electrical disturbance, the heart is denoted to be arrhythmic. The resulting cardiac arrhythmia impairs cardiac efficiency and can be a potential life threatening event.
Cardiac arrhythmias occurring in the atria of the heart, for example, are called supra-ventricular tachyarrhythmias (SVTs). SVTs take many forms, including atrial fibrillation and atrial flutter. Both conditions are characterized by rapid, uncoordinated contractions of the atria. Besides being hemodynamically inefficient, the rapid contractions of the atria can also adversely effect the ventricular rate. This occurs when the aberrant contractile impulse in the atria are transmitted to the ventricles. It is then possible for the aberrant atrial signals to induce ventricular tachyarrhythmias.
Cardiac arrhythmias occurring in the ventricular region of the heart, by way of further example, are called ventricular tachyarrhythmias. Ventricular tachycardia (VTs), for example, are conditions denoted by a rapid heart beat, 150 to 250 beats per minute, that has its origin in some abnormal location with the ventricular myocardium. The abnormal location typically results from damage to the ventricular myocardium from a myocardial infarction. Ventricular tachycardia can quickly degenerate into ventricular fibrillation (VF). Ventricular fibrillation is a condition denoted by extremely rapid, non synchronous contractions of the ventricles. This condition is fatal unless the heart is returned to sinus rhythm within a few minutes.
Implantable cardioverter/defibrillators (ICDs) have been used as an effective treatment for patients with serious ventricular tachyarrhythmias. ICDs are able to recognize and treat tachyarrhythmias with a variety of therapies. To effectively deliver these treatments, the ICD must first identify the type of tachyarrhythmia that is occurring, after which appropriate therapy may be provided to the heart. An inaccurate identification of a detected tachyarrhythmia by the ICD can result in delivery of a high energy therapy, such as defibrillation therapy, when such therapy is unnecessary and undesirable.